So-called three-way catalysts are widely used to purify the hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxides (NOx) present in the exhaust gas output by automotive internal combustion engines. Three-way catalysts are generally able to efficiently purify these three exhaust gas components by oxidation•reduction only when the air/fuel ratio is near the theoretical air/fuel ratio (stoichiometry), and their catalytic activity drops off drastically when the air/fuel ratio is outside the range in the neighborhood of stoichiometry. As a result, the co-use of an oxygen storage material, typified by ceria (CeO2), as a catalyst promoter has been widely practiced in order to broaden the air/fuel ratio range at which the catalyst can exhibit activity. In addition, the use of a ceria-zirconia complex oxide—provided by making ceria and zirconia (ZrO2) into a complex oxide—as a catalyst promoter in place of poorly heat-resistant ceria is also widely practiced in order to improve the heat-resistance properties of the catalyst.
The noble metal catalysts used in three-way catalysts are, for example, platinum (Pt), palladium (Pd), and rhodium (Rh). Among these noble metals, Pd and Pt contribute mainly to the purification function (oxidative purification capacity) for carbon monoxide (CO) and hydrocarbon (HC), while Rh contributes mainly to the purification function (reductive purification capacity) for NOx. In order to bring out the best catalytic activity for these noble metal catalysts with their different properties, porous carriers that are optimal for the individual noble metal catalysts must be sought. Porous carriers that use alumina in combination with a ceria-zirconia complex oxide (for example, Patent Literature 1) have heretofore been widely used as porous carriers for supporting Pd, which is one of the noble metal catalysts. These porous carriers are useful because they can combine the large specific surface area and high durability (particularly heat resistance) possessed by alumina with the oxygen storage/release function possessed by the ceria-zirconia complex oxide.